1. Field of the Invention
The invention pertains generally to an electronic control unit for providing the air/fuel ratio management of an internal combustion engine and is more particularly directed to a hybrid electronic control unit.
2. Prior Art
Electronic fuel schedulers or electronic control units for regulating the air/fuel ratio of an internal combustion engine are conventional in the art. These schedulers provide, from a calculation or electronic computation based upon the operating parameters of the engine, an air/fuel ratio that is considered substantially ideal for the instantaneous conditions sensed.
The "best" air/fuel ratio at which the engine will operate under a given set of operational conditions is normallly a tradeoff between the competing factors of driveability, emissions, and fuel economy. It is generally understood that richer air/fuel ratios are better for power and driveability, a substantially stoichiometric air/fuel ratio the most desirable for emissions, and lean air/fuel ratios the calibration that gives the best fuel economy. The scheduler of desired air/fuel ratios for the electronic control unit can be derived from empirical tests of emissions, driveability, and economy tests and may include areas where the one criterion is more important than the others.
For example, under urban or in city driving conditions emissions are considered of importance because of the congestion of automobilies present in a small area and the amount of pollutants at these slow speeds while at highway or freeway speeds, economy would be the overriding factor of consideration. In addition, for passing or accelerations and to ease starting and warm up situations, rich air/fuel ratios which affect power and driveability must be factored into the scheduling.
Any number of the various engine parameters may be sensed to calibrate the schedule of air/fuel ratios, but the most advantageous method is to measure mass air flow or mass fuel flow and calculate the other from the schedule.
An air/fuel controller having a calibration based upon the speed of the engine and the density of the air as a measurement of mass air flow has been successfully provided by a system disclosed in a U.S. Pat. 3,734,068 issued to J. N. Reddy on May 22, 1973. The disclosure of Reddy is hereby expressly incorporated by reference herein. Reddy discloses a base calibration pulse width that is a function of the RPM of the engine and manifold absolute pressure. The duration of the pulse width signal is used to regulate fuel flow to the engine based upon a schedule. This base calibration is an open loop control of the air/fuel ratio as the operating parameters of the engine are sensed by the controller and a control signal which is the fuel pulse duration or air/fuel ratio change is developed therefrom.
If the air/fuel ratio schedule from which the control signal is calculated or the engine environment to which it is applied is different from the empirically determined optimum system design, then the controller will not perform as required. The difference in engine environments are generally either because of manufacturing tolerances that change the response of different engines, or, as occurs with all mechanical devices, the aging factor which is difficult or impossible to schedule.
It is known in the art that to solve many of the problems faced by open loop fuel schedulers a closed loop integral controller may be effectively utilized. The controllers are termed "closed loop" because they sense the result of an actual air/fuel ratio change and develop a control signal based thereon rather than calculating an air/fuel ratio change from a desired schedule as does the open loop controller. One of the most advantageous of these controller systems is based upon the bi-level output of an exhaust gas composition sensor which indicates whether a rich or lean air/fuel ratio charge has been combusted by the engine. The controller incrementally leans the air/fuel ratio during a rich indication of the sensor and incrementally enrichens the air/fuel ratio during a lean indication of the sensor, thereby causing the system to oscillte in a limit cycle about a desired air/fuel ratio. Illustrative of this type of controller is a U.S. Pat. No. 3,815,561 issued to Seitz which is commonly assigned with the present application. The disclosure of Seitz is hereby expressly incorporated by reference herein.
Previous ECU systems have been essentially all analog, such as Seitz, or all digital, such as disclosed in a copending application Ser. No. 881,321 filed on Feb. 27, 1978 in the name of Hartford et al which application is commonly assigned with the present. The disclosure of Hartford et al is hereby expressly incorporated herein. Although advantageous in design and precise in system control, these systems do not optimumly utilize the input parameters of an internal combustion engine. The parameters used today are a mixture of analog inputs, digital inputs, and logical levels. An all analog system does not readily take advantage of the power of modern digital circuitry in processing the logical level decisions and the reiterative addition and subtraction functions. This is especially true with the advent of small microprocessor chips which may perform many simple logical computations with great rapidity.
An all digital system, however, is also costly from the standpoint of the analog to digital conversions that must be performed to change all the input parameters into digital signals. An all digital system can waste processing time and expensive memory doing multiplications and divisions which can be accomplished more readily in analog form. For real time processing, this can affect the size of the microprocessor chip needed significantly.
It would, therefore, be advantageous to separate those functions in the ECU are more easily accomplished in analog form from those more easily accomplished in digital form. The functions thus separated could be combined or interfaced through a digital to analog converter thereafter since the ECU interface to the engine will be inherently analog as current and voltage signals.
One of the functions more readily accomplished in a digital or microprocessor based function generator is the closed loop integral control which is based on a series of incremental changes in the analog form or representative counts in digital form. An integration of the exhaust gas sensor can be easily accomplished by a microprocessor because digital data can be read into the terminal ports of such a device on a time base. An all digital circuit integration can be further based on a system clock.
There have been a number of closed loop integral controllers proposed in the prior art in analog form. One system disclosed in a copending application 791,092 filed on Apr. 26, 1977 in the name of J. N. Reddy and now U.S. Pat. No. 4,108,266 which is commonly assigned with the present application describes the use of an asymmetric ramp rate. The disclosure of Reddy is hereby expressly incorporated by reference herein. The assymmetry of the ramp rates of the integral controller is utilized to adjust the air/fuel ratio of the ECU slightly off stoichiometric. It is difficult to control steep or fast ramp rates for this type of analog controller with the utmost precision because the discharge of a capactive energy storage device is usually necessary. This discharge process can produce timing errors in an integral waveform. Also, capacitive devices have a tendency to age and change value. Since it is the ratio of the asymmetric ramp rates that provides the precise air/fuel ratio offset from stoichiometric, it is important to retain this ratio as nearly constant as possible. In analog controllers precision components are used to hold tolerances in the system control law.
Other analog integral controller systems have disclosed the utilization of RPM based ramp rates to normalize the authority level of the integral waveform and cascaded analog integral controllers have been proposed to provide combined transient and longer term control. One such cascaded system is disclosed in a U.S. Pat. No. 3,990,411 issued to Oberstadt et al. The disclosure of Oberstadt et al is expressly incorporated by reference herein. Oberstadt further discloses clamping circuitry for discharging the integrating capacitors to a reference level to permit the ECU to run in an open loop mode for certain conditions.